Carbon nanotubes are hexagonal networks of carbon atoms forming seamless tubes with each end capped with half of a fullerene molecule. They were first reported in 1991 by Sumio Iijima who produced multi-layer concentric tubes or multi-walled carbon nanotubes having up to seven walls by evaporating carbon in an arc discharge. In 1993, Iijima's group and an IBM team headed by Donald Bethune independently discovered that a single-wall nanotube could be made by vaporizing carbon together with a transition metal such as iron or cobalt in an arc generator (see Iijima et al. Nature 363:603 (1993); Bethune et al., Nature 363: 605 (1993) and U.S. Pat. No. 5,424,054). The original syntheses produced low yields of non-uniform nanotubes mixed with large amounts of soot and metal particles.
Presently, there are three main approaches for the synthesis of single- and multi-walled carbon nanotubes. These include the electric arc discharge of graphite rod (Joumet et al. Nature 388: 756 (1997)), the laser ablation of carbon (Thess et al. Science 273: 483 (1996)), and the chemical vapor deposition of hydrocarbons (Ivanov et al. Chem. Phys. Lett 223: 329 (1994); Li et al. Science 274: 1701 (1996)). Multi-walled carbon nanotubes can be produced on a commercial scale by catalytic hydrocarbon cracking while single-walled carbon nanotubes are still produced on a gram scale.
Generally, single-walled carbon nanotubes are preferred over multi-walled carbon nanotubes because they have fewer defects and are therefore stronger and more conductive than multi-walled carbon nanotubes of similar diameter. Defects are less likely to occur in single-walled carbon nanotubes because multi-walled carbon nanotubes can survive occasional defects by forming bridges between unsaturated carbon valances, while single-walled carbon nanotubes have no neighboring walls to compensate for defects. Defect-free single-walled nanotubes are expected to have remarkable mechanical, electronic and magnetic properties that could be tunable by varying the diameter, and chirality of the tube.
The electric arch discharge and the laser ablation methods of producing single-walled carbon nanotubes are not practical for producing industrial scale volumes of nanotubes. Both methods involve evaporating carbon atoms from solid carbon sources at greater than 3000° C. thereby setting a limit to the quantity of nanotubes that can be synthesized. Moreover, both methods grow nanotubes in highly tangled form with carbon and metal impurities. The nanotubes produced by these methods are difficult to purify, manipulate and assemble for building nanotube devices. The chemical vapor deposition method, on the other hand, is scalable and represents possibly the best method for the large scale production of nanotubes.
Presently, there are two types of chemical vapor deposition for the large scale syntheses of single-walled carbon nanotubes that are distinguishable depending on the form of supplied catalyst. In one, the catalyst is embedded in porous material or supported on a substrate, placed at a fixed position of a furnace, and heated in a flow of hydrocarbon precursor gas. Cassell et al. (1999) J. Phys. Chem. B 103: 6484-6492 studied the effect of different catalysts and supports on the synthesis of bulk quantities of single-walled carbon nanotubes using methane as the carbon source in chemical vapor deposition. They systematically studied Fe(NO3)3 supported on Al2O3, Fe(SO4)3 supported on Al2O3, Fe/Ru supported on Al2O3, Fe/Mo supported on Al2O3, and Fe/Mo supported on Al2O3—SiO2 hybrid support. The bimetallic catalyst supported on the hybrid support material provided the highest yield of the nanotubes. Su et al. (2000) Chem. Phys. Lett. 322:321-326 reported the use of a bimetal catalyst supported on an aluminum oxide aerogel to produce single-walled carbon nanotubes. They reported preparation of the nanotubes is greater than 200% the weight of the catalyst used. In comparison, similar catalyst supported on Al2O3 powder yields approximately 40% the weight of the starting catalyst. Thus, the use of the aerogel support improved the amount of nanotubes produced per unit weight of the catalyst by a factor of 5.
In the second type of carbon vapor deposition, the catalyst and the hydrocarbon precursor gas are fed into a furnace using the gas phase, followed by the catalytic reaction in a gas phase. The catalyst is usually in the form of a metalorganic. Nikolaev et al. (1999) Chem. Phys. Lett. 313:91 disclose a high-pressure CO reaction (HiPCO) method in which carbon monoxide (CO) gas reacts with the metalorganic iron pentacarbonyl (Fe(CO)5) to form single-walled carbon nanotubes. It is claimed that 400 g of nanotubes can be synthesized per day. Chen et al. (1998) Appl. Phys. Lett. 72: 3282 employ benzene and the metalorganic ferrocene (Fe(C5H5)2) delivered using a hydrogen gas to synthesize single-walled carbon nanotubes. The disadvantage of this approach is that it is difficult to control particles sizes of the metal catalyst. The decomposition of the organometallic provides the metal catalyst having variable particle size that results in nanotubes having a wide distribution of diameters. Further, the decomposition of the metalorganic precursor forms carbon structures that are not desired. This is presumably because the temperature at the inlet of the reactor is very low in comparison with the decomposition temperature of the catalyst material, and the heat conductivity of the gas is very low. Therefore, the catalyst is gradually heated and slowly decomposed, with the possibility that they are completely covered with carbon layers before they form catalyst particles suitable for nanotube growth.
In another method, the catalyst is introduced as a liquid pulse into the reactor. Ci et al. (2000) Carbon 38:1933-1937 dissolve ferrocene in 100 mL of benzene along with a small amount of thiophene. The solution is injected into a vertical reactor in a hydrogen atmosphere. The technique requires that the temperature of bottom wall of the reactor had to be kept at between 205-230° C. to obtain straight carbon nanotubes. In the method of Ago et al. (2001) J. Phys. Chem. 105: 10453-10456, colloidal solution of cobalt:molybdenum (1:1) nanoparticles is prepared and injected into a vertically arranged furnace, along with 1% thiophene and toluene as the carbon source. Bundles of single-walled carbon nanotubes are synthesized.
The synthesis of long strands of ordered single-walled carbon nanotubes by catalytic chemical vapor deposition has been reported by Zhu et al. (2002) Science 296:884-886. They utilize the floating ferrocene catalyst in a vertical furnace to catalytically pyrolyze n-hexane, where n-hexane is combined with thiophene and hydrogen. Typically, thiophene and ferrocene are dissolved in n-hexane and sprayed into the hydrogen stream from the top of a vertical furnace heated to the pyrolysis temperature of 1423K. The gas flow carries the nanotube strands downstream, and the nanotubes can be collected at the bottom of the furnace at the reported yields of about 0.5 g/hour.
The two-step method of preparing a colloidal solution and injecting it into the reactor has the disadvantages of forming non-desired carbon structures due to the decomposition of the organic surfactants and a low yield of the single-walled carbon nanotubes. Thus, the presently available methods of synthesizing carbon nanotubes produce bulk amounts of carbon nanotubes that are generally tangled and kinked. Further, the nanotubes can have molecular level structural defects that can adversely impact their properties.
Thus, there is a need for methods for bulk, industrial scale synthesis of carbon nanotubes. Preferably, the method allows for the growth of carbon nanotubes of a desired type, such as single-wall nanotubes, and with little or no impurities.